<script>
    var i = 1;
    var j = 1;

    // () => 自加和自减 => 算术运算 => 关系运算(比较运算) => 逻辑运算 => 赋值运算
    var sum = 20 + i++ + 'abc' || 123 - ++i + ++j + ++j + '123' == 123 * 2/4;   // '21abc' 


</script>